From EP 0732036 A1 an automatic regulation circuitry for hearing aids is known for a programmable hearing aid wherein an electronic signal processing circuit has a regulation circuit for continuously determining or calculating one or several percent values of the input signal based on a continuous analysis and evaluation of the frequency and/or amplitude distribution of the input signal. These percent values are directly or indirectly used as control signals for regulating the amplification and/or the frequency response of the electronic signal processing circuit.
Hearing aid level detectors are also known from U.S. Pat. No. 4,531,229 and U.S. Pat. No. 5,144,675 wherein a peak value detecting circuit is combined with an average value detecting circuit. The peak value detecting circuit provides adjustment with short time delays and the average value detecting circuit provides adjustment with long time delays. Heavy sound levels of short duration will quickly excite the peak value detecting circuit and provide a quick gain reduction, but after a heavy sound of longer duration which disappears, the gain is adjusted slowly as a function of the decreasing mean value and during a time interval thereafter there will be an insufficient amplification of weak signals.
From WO 99/34642 automatic gain control in a hearing aid is effected by detecting an input sound level and/or an output sound level and adapting the output sound level supplied by the hearing aid in response to the detected sound level by controlling the gain of the hearing aid towards an actual desired value of the output sound level. The gain control is effected at increases and decreases, respectively, of the input sound level by adjusting the gain towards the actual desired value with an attack time and a release time, respectively, which are adjusted in response to the detected sound level to a relatively short duration providing fast gain adjustment at high input and/or output sound levels and to a relatively long duration providing slow gain adjustment at low input and/or output sound levels.
From U.S. Pat. No. 6,198,830 a compressor and accompanying level detector is known, wherein the time constants of the level detector are set after conducting and analysis of the modulation frequency of the input signal in order to classify the input signal.